1. Field of the Invention
The present invention relates generally to devices for installing wells. In particular, the present invention relates to devices for assembling and installing groundwater-monitoring wells and other wells having prepacked screens.
2. Description of the Related Art
Conventional groundwater monitoring wells are typically constructed through hollow stem augers or probe rods by lowering slotted PVC pipe (screen) to depth on the leading end of a string of threaded PVC riser pipe. A filter pack is then installed by pouring clean sand or other granular material of known particle size through the tool string annulus until the slotted section of the PVC pipe is sufficiently covered.
Installing the entire filter pack through the tool string annulus becomes a delicate and time-consuming process when performed with small-diameter, direct push tooling. Granular material must be poured very slowly to avoid bridging between the riser pipe and surrounding probe rod. When bridging does occur, considerable time can be lost in attempting to dislodge the granular material or possibly pulling the tool string and starting over.
Prepacked screens have been developed to improve the monitoring well installation process. An example of a conventional prepacked screen assembly 50 is shown in FIGS. 1 to 4 of the accompanying drawings. As shown in FIG. 1, a conventional monitoring well with a prepacked screen assembly 50 can be installed using direct push tooling having an expendable point 60 at the lower end, a PVC plug 61 at the bottom of the screen assembly 50, a PVC riser 62 above the screen assembly 50, and a locking plug 63 with a padlock 64 at the upper end of the PVC riser 62. After placement in the soil bore, the prepacked screen assembly 50 is covered by a grout barrier 65, a Bentonite well seal 66 above the grout barrier 65, and a neat cement grout barrier 67 above the Bentonite well seal 66. A concrete pad 68 and a flush mount well cover 69 are placed over the well at the soil surface 70 to complete the installation.
The prepacked screen assembly 50 in the conventional monitoring well typically includes a standard, slotted PVC well casing 51 surrounded by a stainless steel mesh 52. Granular material S is packed between the slotted PVC well casing 51 and the stainless steel mesh 52 by the manufacturer before the product is delivered to the consumer. Since the granular material S is packed around the slotted PVC well casing 51 before it is installed, the use of prepacked screen assemblies 50 guarantees that granular material S will be located directly around the well casing 51, and that the slotted PVC well casing 51 will be concentrically located in the granular material S. This makes well installation quicker and more efficient than conventional methods. It has been shown that the quality of samples obtained from wells installed with prepacked screen assemblies 50 correlates well with samples obtained from conventionally installed wells, but at less cost.
Prepacked screen assemblies 50 greatly decrease the amount of time, cuttings and loose sand or other granular material required for well installation as each screen assembly includes the necessary filter pack. Granular material must still be delivered through the casing annulus to provide a minimum 2-foot grout barrier, but this volume is significantly less than for the entire screened interval.
A common method for building prepacked screen assemblies 50 involves using a smooth-bored PVC bushing 53, held axially in place on the slotted pipe 51 by a stainless steel clamp 54, as shown in FIGS. 3 and 4. Although quite common, this method has limitations, most notably with respect to the time to assemble, as well as the permanence of the fastening. Considerable time is spent in assuring that the bushing 53 is correctly located along the length of the slotted pipe 51. The bushing 53 is then secured by placing and crimping the stainless steel clamp 54 on the slotted pipe 51 behind the bushing 53 to act as a shoulder, preventing it from moving axially in one direction. The stainless steel band 54 does not prevent the bushing 53 from moving in the other direction, nor is the bushing 53 fastened so securely that it cannot be made to slide out of position. The stainless steel mesh 52 is secured at each of its ends to the outer surfaces of the respective bushings 53 using stainless steel clamps 55.
Thus, there is a need in the industry for an improved device for assembling prepacked screens for monitoring wells.